1. Field of the Invention
The present invention generally relates to semiconductor techniques for printing. Nandakumar Vaidyanathan
2. General Background and State of the Art
There are currently several dominant techniques used in computer based and commercial printing (non-impact printing).
A large portion of Personal Computer (PC) based printing is based on Ink Jet technology, or “Drop on Demand” methods where the image to be printed is constructed on an appropriate printing medium such as paper, plastic, textiles, printing plates and even silicon based substrates using print heads which eject drops of ink at the appropriate location on the printing medium. Since the ejection of ink occurs at the time the image is being printed this is often called “Drop on Demand” printing. The ink ejection mechanism may be controlled using piezo electric mechanisms or thermal mechanisms (ink jet or bubble jet). These printing methods rely on electronics that reside on the computer and on the printing equipment to deposit the ink on the printing medium. Since the entire image is constructed on a drop-by-drop basis, this can be a rather slow process.
Another kind of commercial printing that is carried out using the ink-jetting technique is called the Continuous Ink-Jetting Method. In this method, a continuous jet of ink is squirted through space, and using electrostatic deflector plates, the ink is selectively directed at the appropriate medium through a mesh, leading to deposition of dots to create patterns. The unused ink is directed through another channel and is recycled. This is the basis of the Continuous Ink Jetting technique and this process uses both charged and uncharged inks.
Another popular PC based printing method is “Laser Jet” or “Laser Writing” which is based on electrophotography. This method originated from Xerographic techniques for replication of images. In the original xerographic technique, a charged drum (photoconductive drum) is optically exposed to the image to be duplicated. Based on the image, charges are removed on the photoconductive drum using either a laser beam, or any other light source of appropriate spectral content and energy such as light emitting diodes (LED's). Specially charged ink, called toners, which could be either a fine powder or a liquid, are attracted to the locations on the photoconductive drum, which have the opposite electrical polarity. From the photoconductive drum, these charged particles are then transferred to the printing medium. In this method of printing, the contents of the entire image can be transferred to a photoconductive drum, and then the transfer effected to the printing media in a single step. This method of image transfer is therefore faster than the “Drop on Demand” technique previously described.
Another printing technology used in the commercial printing world, called magnetography, is similar to electrophotography, but uses magnetic fields instead of electrostatic fields to propel charges.
Perhaps the most dominant technology in the commercial printing world is based on lithography. Lithography involves a plate or an intermediate medium, on which the image to be printed is either exposed or engraved using a variety of techniques such as photography, laser ablation, thermal ablation and more recently ink jet based techniques. The areas of the printing plate have areas which accept ink (olephilic—oil loving) and areas, which accept water (hydrophilic). In general, the oil loving areas of the image do not accept water and the water loving areas do not accept ink. As the lithographic printing ink is an emulsion of pigments and water, the ink and water selectively migrate to their respective locations on the printing plates. Once the ink and water have migrated to their respective locations, it is then transferred to the medium being printed or to an intermediate cylinder called an offset cylinder and from the offset cylinder the image is deposited on the final medium.
There are four other processes, namely flexography, gravure, letterpress and screen printing.
The above-mentioned technologies are fairly well established. They have great advantages in their respective niches. However, there are significant disadvantages with each of the methods.
For example, as previously mentioned, ink jet based printers are quite slow. There are high costs associated with electrostatic printing processes for commercial printing, due to low throughput and inability to provide more than a certain number of copies (40,000 copies with current technology) on an electro-photography based machine, before the photoconductor drum is rendered useless for any other more reproduction. In lithographic printing, primary costs include use of expensive printing plates or spools, and high costs for recycling and disposal of environmentally unfriendly chemicals. Furthermore, the imaging or pre-imaging equipment used in the commercial printing world can be quite large and bulky.
Most commercial printing technology also involves disposable pieces. For example, lithographic printing involves using a new printing plate for every image printed. There are also inks that need to be poured and replenished, if one wants to make a large number (many thousands) of copies. With xerography, a new printing plate is not used each time. However, the same large number of copies cannot be made because the charges wear off and need to be replenished. In addition, the photoconductive drums lose sensitivity to spectral content after multiple usage.
Finally, personal printers such as inkjet and laser printers utilize ink cartridges, which need to be replaced on a regular basis. Much of the money made in the personal printing market is by consumables such as ink cartridges, toner, drums, and printing plates.